1. Field of the Invention
This invention relates generally to cylinders, and more specifically to an overload safety device.
2. Description of the Prior Art
It is generally well known in the art to provide overload safety devices to prevent damage to interacting machine parts or tools. In such devices illustrated in U.S. Pat. Nos. 3,178,983 and 3,190,141, when the force generated between a tool and work piece exceeds a predetermined force a part of the device gives way providing a fluid escape chamber for the fluid permitting relative movement between the plunger and cylinder. One of the disadvantages of such overload safety devices is that the preloading pressure significantly must be exceeded before the overload device operates, and hence it is possible that some damage may result before the fluid release takes place. Another disadvantage of this overload safety device is that the plunger, due to fluid pressure, gravity and other forces occurring during high speed, reciprocating motion of the plunger, will strike an obstruction with great force before the release takes place causing damage to the tool or parts. Also, in U.S. Pat. No. 2,033,181, a. rotary shear is disclosed in which the shear elements yield and recede from each other in the case of undue load or resistance. In this prior art mechanism, when the oil pressure exceeds a predetermined force, a valve releases relieving the pressure on a piston which in turn automatically opens another valve permitting the oil to bypass the power cylinder and prevent any further increase in pressure. A disadvantage of this overload device is that the valves are relatively slow acting, and if used in a high speed reciprocating power device permit several strokes of the device to occur before the pressure is sufficiently released, thereby resulting in damage to the parts. In U.S. Pat. No. 2,783,838, another form of safety device for use in power presses is disclosed utilizing an electronic control means to terminate the downward movement of the platen in response to the reduction of electrical resistance between conductive members. A disadvantage of this safety device is that the electrical components are not always reliable, and such a safety device further reacts too slowly since electrical solenoids are involved for actuating the fluid control valves.